COSI Blog
01
June
2012

Venus Crossing!

Venus Crossing!

In Roman mythology, Venus was the goddess of love. From Earth, the lovely planet Venus is the brightest object we can see besides the Sun and the Moon. We know today that Venus itself is not so lovely a place, with clouds of sulfuric acid and a surface hot enough to melt lead. But because Venus is between the Earth and the Sun, every so often, something very special happens. Venus passes directly between our planet and our star. We call this passage a transit.

Imagine yourself as a space alien, looking down on our solar system. In the center is the Sun, dwarfing all else. A few million miles* from the Sun you see some orbiting rocks. Two of them, almost the same size, are the second and third rocks from the Sun. The orbits of these planets are almost (but not quite) circles, and they’re almost (but not quite) in the same plane. If the planets were exactly in the same plane, it would be as if they were marbles rolling about on a flat plate as they circle the Sun. If that were the case, then every time Venus overtook the Earth in its orbit (something that happens at least once every year) people on Earth would see Venus pass in front of the Sun. But because our orbits are tilted, this perfect passage (or transit) occurs only when conditions are just right.

That turns out to happen only every 110 years or so. It’s one of the rarest predictable astronomical events we know. The strange thing is, whenever a transit occurs, it happens twice, around eight years apart. Venus last made a transit in 2004. Its next transit is right around the corner, on June 5, 2012. Such a transit won’t happen again until (ready for this?) December 11, 2117! Of course, you can’t see this event in the sky, because looking at the Sun is very dangerous. One of the best ways to “see” what Venus and the Sun are up to on June 5 is to use a pinhole camera. One of our science museum colleagues in Australia has created this great Pinhole Camera Kit for free online – you just print it out and follow the instructions!

There are lots of good warnings on the cards about not looking at the Sun and so on, but one thing the card doesn’t tell you is how a pinhole camera works! So here goes:

A pinhole camera works because light moves in straight lines. Even light from the Sun, 93 million miles away, travels in a straight line from the Sun’s surface to you. When this light reaches your pinhole, it passes through the pinhole and hits the card on the other side. One strange feature of the pinhole camera is that at the pinhole, all the straight lines cross. That means light from the top of the Sun ends up on the bottom of your image, while light from the bottom of the Sun ends up on top. So on your pinhole camera, you’re looking at the Sun upside down!

If you set up your pinhole camera just right (and if the clouds stay away!), at a little after 6:00 pm you’ll see a tiny dot appear on the right side of the Sun (remember, it would be the left side in the sky, but your pinhole camera flips everything). That tiny dot is the planet Venus, making its way across the face of the Sun! Enjoy this view while you can – it won’t happen again for another 105 years!

Did You Know?
In the 1700s, astronomers knew the order of the planets. They also knew that Earth was 1.4 times as far from the Sun as Venus. But nobody knew the actual distance of the Earth (or any other planet) from the Sun. It wasn’t until astronomers carefully measured and timed the transits of 1761 and 1769 that we learned the true scale of the solar system. Because of those careful measurements over 200 years ago, we know now that the Earth is 93 million miles (over 11,000 Earth diameters) from the Sun.

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